Use a piece in a new, quirky or unexpected manner. This way, you may make that piece a focal point or use it to build the theme of the room. Artwork or a couch are good choices to stand out as they are larger or more significant elements, to which the eye will be immediately drawn.
Find a theme other than a furniture style to unite the collection. Basically, you should look for a commonality among pieces. Some people use color or finish, but a particular ornamentation or detail, pattern, fabric or intended use of the items (as in mixed and matched bookcases) also works.
Wooden antique pieces may be the easiest choices for adding to any space. The quality of hardwoods and finishes tend to complement almost any decor and either set off more vibrant pieces or give a touch of elegance to a modern space.
Adding small touches may help pull the room together. For example, throw pillows bring the colors of two diverse seatings together. Layers add depth to your space, which will be absorbed in stages and provoke visual interest.
If you are afraid of making a mistake, try maintaining your living room with one primary choice of design and use a secondary style to add unique elements. This will prevent the room from looking hodgepodge, but bring enough variation to catch the eye and allow you to express your personality.